The present invention relates to an electro-photographic image formation apparatus using a liquid toner.
The electrophotographic image formation apparatus has been reevaluated in recent years since the apparatus has the following advantages as compared to the apparatus using a solid toner. More specifically, extremely small particles of a sub-micron order can be used as toner particles, so that a high quality image equal to, e.g, an off-set printing level, can be realized. Furthermore, an image can be obtained with a sufficient density by fixing a small amount of the liquid toner on a paper sheet at a relatively low temperature, it is therefore possible to realize an economical and energy-saving apparatus.
FIG. 1 shows an electrophotographic image formation apparatus using a liquid toner. Now, we will explain a case where a full color image is formed using four color liquid toners. In FIG. 1, a photosensitive drum 1 is constituted by forming an organic or an amorphous silicon photosensitive layer on a drum formed of a conductive base body. After the photosensitive drum 1 is uniformly charged by a first charger 2a, a first light exposure 3a is performed corresponding to a modulated image, with the result that a first electrostatic latent image is formed on the surface. Thereafter, the electrostatic latent image is developed by a developing apparatus 4a storing a first liquid toner. Subsequently, the photosensitive drum 1 is charged by a second charger 2b and subjected to a second light exposure 3b. In this manner, a second electrostatic latent image is formed. The second electrostatic latent image is then developed by a second developing apparatus 4b. Third and fourth development processes are further performed in the same manner as above. Finally, a full color toner image is formed on the photosensitive drum 1. The toner image formed on the photosensitive drum 1 is transferred to a transfer medium 5 by application of an electric field. The toner image transferred onto the transfer medium 5 is further transferred onto a paper sheet 7 fed between the transfer medium 5 and a pressure roller 6. Before a next image formation process, the toner remaining on the photosensitive drum 1 is removed by a cleaner 8. As described, the transfer medium 5 has a function of receiving the toner developed on the photosensitive drum 1 and transferring the toner to an image formation medium such as a paper sheet.
In the meantime, a printing press employed in the printing industry is suitable for use in printing a number of identical images by using an original plate. In a case of the off-set printing, an image (ink layer) formed on a flat plate is transferred to a blanket serving as a transfer medium and further transferred from the blanket to an image formation medium, such as a paper sheet, to thereby obtain a printed matter. In this case, no electric field is applied between the flat plate and the blanket. In addition, since entirely identical printed matters are obtained, offset ink is not necessarily transferred to the blanket with a transfer efficiency of 100%. Accordingly, the blanket may well have a necessary ink release property, so that endurance is required as the most important property.
In contrast, in an image formation apparatus called "on-demand type" for printing a different image on each paper sheet, the image transfer must be performed always with a transfer efficiency of 100% since a different image is output on each paper sheet. Alternatively, toner remaining on the transfer medium must be removed by performing a cleaning operation before a next image transfer process.
In a conventional electrophotographic image formation apparatus using a liquid toner as shown in FIG. 1, used is the transfer medium which is formed of an inner layer formed of a conductive rubber layer and a surface layer formed of a silicone-based binder resin filled with an insulating filler.
However, in the case of the transfer medium of this type, when the toner image on the photosensitive drum is transferred to the transfer medium by application of the electric field, a transfer efficiency close to 100% cannot be obtained since a sufficient electric field is not applied to the high-resistance surface layer due to the presence of the insulating filler. Since a carrier solvent for the liquid toner is attached to the transfer medium, the attached solvent permeates into the underlying conductive rubber layer if the surface layer has a high absorbency of the solvent. As a result, the overall structure of the surface layer and the conductive rubber layer swells, with the result that dimensional change, for example, change in rubber thickness, occurs. In addition, mechanical strength reduces, with the result that the surface layer is sometimes peeled off. Such a mechanical deformation causes inconsistencies in density in the transferred image and is responsible for poor image quality.